In optical networks such as fiber to the home (FTTH), cable television (CATV), etc., a PON type optical transmission system can be used in which an optical transmission line connected to a center is split by a passive splitter and the split optical transmission lines are laid on a plurality of subscribers. This optical transmission system is also called a Passive Double Star (PDS) system.
In such an optical transmission system, the splitter is connected to an optical fiber drawn out to users from an optical subscriber unit (OSU) in an optical line termination (OLT) of the center, and optical network units (ONUs) of a plurality of users are coupled to a plurality of optical transmission lines split by the splitter through optical fibers.
In a PON type or a PDS type optical transmission system in which an OLT is connected to a plurality of ONUs through optical fibers and a splitter, it is known that an optical repeater can be connected to the optical transmission line between OLT and the splitter in order to lengthen the optical transmission line.
Furthermore, a system can be constructed in such a manner that a multiplexer/demultiplexer (MUX/DEMUX) and an optical repeater are connected to an existing optical transmission system to perform wavelength division multiplexing (WDM), as disclosed in Japanese Patent Laid-Open Publication No. 2002-261697. The optical repeater has a configuration shown in FIG. 15.
A wavelength converter 101 includes first and second 3 dB couplers 102 and 103 respectively mounted at input and output stages of the OLT and the ONU. A downstream signal system includes a light-receiving element 104, a receiving circuit 105, a driving circuit 106, a light-emitting element 107, and an isolator 108, which are connected in order between the first and second 3 dB couplers 102 and 103 toward the signal transfer direction. An upstream signal system includes a light-receiving element 109, a receiving circuit 110, a driving circuit 111, a light-emitting element 112, and an isolator 113, which are connected in order between the first and second 3 dB couplers 102 and 103 toward the signal transfer direction.
In the converter, the light-receiving elements 104 and 109 respectively convert optical signals output from the first and second 3 dB couplers 102 and 103 into electrical signals, and the receiving circuits 105 and 110 perform a 3R or 2R process on the electrical signals. Then, the driving circuits 106 and 111, the light-emitting elements 107 and 112 convert the signals subjected to the 3R or 2R operation into optical signals and output the converted optical signals to the isolators 108 and 113. Here, 3R represents reshaping, regeneration and retiming functions and 2R represents regeneration and retiming functions.